Hybrid propulsion for fuel-cell cars

ABSTRACT

The invention relates to a hybrid drive for an electric vehicle having a fuel cell, an energy store, an electric traction motor and electrical auxiliary loads. The invention proposes providing two separate circuits, provided with switching devices, for selectively connecting the electric traction motor and the electrical auxiliary loads to the fuel cell or to the energy store, and providing a switchable connecting line between the fuel cell and the energy store.

TECHNICAL FIELD

The invention relates to a hybrid drive for an electric vehicle having afuel cell and an energy store.

DISCUSSION

U.S. Pat. No. 5,334,463 A1 discloses a hybrid drive, comprising a fuelcell, a battery, an electric traction motor and electrical auxiliaryloads, the fuel cell and the battery being connected in series with theelectric traction motor and the electrical auxiliary loads by means of acommon circuit, With sufficient voltage, the battery can be charged bythe fuel cell.

Such an arrangement having only one circuit has the disadvantage thatthe larger of the two voltage sources, that is to say fuel cell orbattery, determines the maximum driving power, since it is not possibleto connect two current sources having different voltages to one load.This means that, at full load, the electric traction motor is suppliedby the fuel cell or by the battery, depending on which of the twovoltage sources provides greater power.

In addition, U.S. Pat. No. 5,519,312 A1 discloses a hybrid system,comprising a fuel cell and a superconductive magnetic store (SMES) whichare connected to an electrical load by means of a converter. Suitableswitches and a control unit can be used to connect the fuel cell to theSMES store and to connect the fuel cell and/or the SMES store to theelectrical load.

A further hybrid system, comprising a fuel cell and two batteriesconnected in parallel therewith, is disclosed in EP 0 136 187 A2.Suitable switches can be used to isolate and connect the fuel cell andthe batteries from/to the circuit.

Finally, U.S. Pat. No. 5,166,584 A1 discloses an electric vehicle havinga hybrid system which comprises a battery and an electrical generatorand in which, in overload mode, the current supply for auxiliary loadsis reduced in order to ensure sufficient current supply for the tractiondrive.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a hybrid drive for electricvehicles having improved driving powers.

The object is achieved by the characterizing features of Patent claim 1.

The hybrid drive according to the invention has the advantage that itpermits variable control, which, in particular, improves the maximumdriving power. In the first illustrative embodiment, the auxiliary loadscan be supplied by the energy store at full load. This means that themaximum fuel cell power is available for driving the vehicle. In all theother illustrative embodiments, the electric traction motor can bedriven by the fuel cell and the energy store at full load, which meansthat the maximum driving power is given as the sum of fuel cell andenergy store power.

In addition, in braking mode, the electrical auxiliary loads can besupplied by the fuel cell, which means that the energy store can also becharged at low brake powers, that is to say even below the powerrequirement of the electrical auxiliary loads.

The use of a DC/DC converter in the connecting line between the twocircuits also allows the energy store to be charged when the vehicle isat a standstill, irrespective of the voltage or the load state of thefuel cell. In addition, the energy store can also be charged anddischarged while the vehicle is in motion, irrespective of the voltageor the load state of the fuel cell. Nevertheless, undesirable lossesthrough the DC/DC converter arise only when the energy store is beingcharged or when the electric traction motor is being operated by theenergy store. Since the fuel cell is connected to the electric tractionmotor without the interposition of a DC/DC converter, no such lossesarise when the electric traction motor is being operated by the fuelcell.

Additional benefits and advantages of the present invention will becomeapparent to those skilled in the art to which this invention relatesfrom a reading of the subsequent description of the preferred embodimentand the appended claims, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings, which form an integral part of the specification, are tobe read in conjunction therewith, and like reference numerals areemployed to designate identical components in various views.

FIG. 1 shows a basic illustration of a hybrid drive having two circuits,

FIG. 2 shows the hybrid drive from FIG. 1 with a DC/DC converter betweenthe fuel cell and the energy store,

FIG. 3 shows the hybrid drive from FIG. 1 with a second electrictraction motor, and

FIG. 4 shows a basic illustration of a further hybrid drive having aDC/DC converter.

DETAILED DESCRIPTION

The hybrid drive shown in FIG. 1 contains a fuel cell 1, an energy store2, an electric traction motor 3 and electrical auxiliary loads, denotedas a whole by 4. The fuel cell 1 is any desired device for producingelectrical energy by chemical conversion of an arbitrary fuel. Such fuelcells for vehicle applications, for example fuel cells having a protonexchange membrane, are known to the expert and are therefore notexplained in more detail below.

The energy store 2 is preferably a battery for storing electricalenergy. However, other energy stores, for example capacitors or flywheelstores, can also be used. Any desired electric motors, for exampleasynchronous motors or reluctance motors, may be used as the electrictraction motor 3. The electric traction motor 3 should preferably alsobe able to be operated as a generator. The term electrical auxiliaryloads 4 covers all electrical loads, including a vehicle power supply,which are not used directly for driving the vehicle.

To supply the electrical loads 3, 4, two separate circuits 5 a, 5 b areprovided. The first circuit 5 a contains a line 7, provided with aswitching device S6, between the fuel cell 1 and the electric tractionmotor 3, and also a line 8 which branches off from the line 7 betweenthe fuel cell 1 and the switching device S6. The second circuit 5 bcontains a further line 10, which is provided with a switching device S9and is connected to the energy store 2. The lines 8, 10 can beselectively connected to the electrical auxiliary loads 4 by means of achangeover device S11. In addition, the second circuit 5 b contains aline 13 which is provided with a further switching device S12, branchesoff from the line 10 between the switching device S9 and the changeoverdevice S11 and connects the second circuit 5 b to the first circuit 5 abetween the switching device S6 and the electric traction motor 3.

To control the switching devices S6, S9, S11, S12, a control unit 14 isadditionally provided which receives a multiplicity of information itemsabout the operating state of the vehicle as input data. By way ofexample, this is information about the voltage or the load state of thefuel cell 1, the speed of the electric traction motor 3, the energyrequirement of the electrical auxiliary loads 4, the voltage or thecharge state of the energy store 2, a signal for detecting a brakingoperation, or the temperature of the fuel cell or energy store. Thecontrol unit 14 uses this, and possibly other, information to ascertainthe operating state of the vehicle. A selection of possible operatingstates (FC=fuel cell 1, AL=auxiliary loads 4, store=energy store 2) withassociated switch positions for the hybrid drive shown in FIG. 1 isshown in Table 1.

In driving mode, it is possible to distinguish between three states,where, for low and medium load, the electric traction motor 3 and theelectrical auxiliary loads 4 are connected to the fuel cell 1 by meansof the first circuit 5 a. In this case (see state {circle around (1)}),the switching device S6 is closed, and the electrical auxiliary loads 4are connected to the fuel cell 1 by means of the changeover device S11and the line 8. The switching device S12 is open, which means that theelectric traction motor 3, the fuel cell 1 and the electrical auxiliaryloads 4 are isolated from the energy store 2. The position of theswitching device S9 is arbitrary. The switching device S9 canadditionally be used as a disconnecting switch in order to isolate theenergy store 2 from the power supply in critical operating states.

TABLE 1 Possible states of the hybrid drive shown in FIG. 1 State/switchposition S6 S9 S11 S12 {circle around (1)} Driving with FC, AL on FC 10/1 0 0 {circle around (2)} Driving with FC, AL on store 1 1 1 0 {circlearound (3)} Driving with store, AL on store 0 1 1 1 {circle around (4)}Braking, AL on FC 0 0/1 0 0 {circle around (5)} Braking, AL on FC, storecharging 0 1 0 1 {circle around (6)} Braking, AL on store, storecharging 0 1 1 1 {circle around (7)} Braking, AL on generator 0 0 1 1{circle around (8)} Standstill, AL on FC 0 0/1 0 0 {circle around (9)}Standstill, AL on store 0 1 1 0

At full load (see state {circle around (2)}), the switching device S6 islikewise closed, which means that the fuel cell 1 is connected to theelectric traction motor 3 by means of the line 7. In contrast to thefirst case, the electrical auxiliary loads 4 are not connected to thefuel cell 1 by means of the line 8, however, but rather are connected tothe energy store 2 by means of the changeover device S11, the line 10and the closed switching device S9. The switching device S12 is open,which means that the circuits 5 a, 5 b are isolated from one another.This state has the advantage that the fuel cell 1 is not loaded with theelectrical auxiliary loads 4. Instead, the whole fuel cell power isavailable for the electric traction motor 3. The supply of theelectrical auxiliary loads 4 is undertaken by the energy store 2.Consequently, in this arrangement, the maximum driving power for a givenfuel cell power is increased by the unneeded power of the electricalauxiliary loads 4.

A further possible driving state, for example when the vehicle isstarted or when there is a very low load, describes state {circle around(3)}. In this case, both the electric traction motor 3 and theelectrical auxiliary loads 4 are supplied with voltage by the energystore 2. In contrast to state {circle around (2)}, only the switchingdevice S6 is open in this case, and the switching device S12 is closed.This state is advantageous while the fuel cell 1 is not yet operationalor else is turned off to improve efficiency.

In braking mode, it is possible to distinguish between four operatingstates. In order to be able to recover energy in braking mode, theelectric traction motor 3 must also be able to be operated as agenerator. In the text which follows, however, an electric tractionmotor/generator unit will also always be referred to only as an electrictraction motor 3. In state {circle around (4)}, the electrical auxiliaryloads 4 are connected to the fuel cell 1 by means of the changeoverdevice S11. The switching devices S6 and S12 are open, and the positionof the switching device S9 is arbitrary. In this state {circle around(4)}, the braking energy is not utilized. In order to charge the energystore 2 even at low brake powers, that is to say below the powerrequirement of the electrical auxiliary loads 4, it is possible tochange to state {circle around (5)}. To this end, the switching devicesS9 and S12 are closed, which means that, although the electricalauxiliary loads 4 are still connected to the fuel cell 1, the electrictraction motor 3 is additionally connected to the energy store 2 bymeans of the second circuit 5 b.

In state {circle around (6)}, preferably during a severe brakingoperation, the energy store 2 can be charged by the electric tractionmotor 3 via the second circuit 5 b when the switching devices S9 and S12are closed. At the same time, the electrical auxiliary loads 4 areconnected to the electric traction motor 3 or to the energy store 2 bymeans of the changeover device S11. The switching device S6 is open,which means that the fuel cell 1 is completely decoupled from the secondcircuit 5 b. In state {circle around (7)}, the switching devices S6 andS9 are finally open, which means that both the fuel cell 1 and theenergy store 2 are decoupled from the power supply. The electricalauxiliary loads 4 are connected directly to the electric traction motor3 by means of the changeover device S11 and the switching device S12.

When the vehicle is at a standstill, the electrical auxiliary loads 4can finally be connected to the fuel cell 1 (state {circle around (8)})or to the energy store 2 (state {circle around (9)}) . In state {circlearound (8)}, the fuel cell 1 is connected to the electrical auxiliaryloads 4 by means of the changeover device S11 and the line 8. Theswitching devices S6 and S12 are open, and the position of the switchingdevice S9 is arbitrary. If the fuel cell 1 is to be disconnected whenthe vehicle is at a standstill, then the electrical auxiliary loads 4are connected to the energy store 2 by means of the changeover deviceS11, the line 10 and the switching device S9 (state {circle around(9)}). Since the fuel cell 1 and the electric traction motor 3 are notin operation, the switching devices S6 and S12 are preferably open.

As the present description reveals, the arrangement having two separatecircuits 5 a, 5 b can produce variable switching states, with a suitableswitch position enabling two independent circuits 5 a, 5 b havingdifferent voltage levels to be operated at the same time. In thisarrangement, the fuel cell 1 can be connected to the electric tractionmotor 3 and/or to the electrical auxiliary loads 4 in each case. Thesame applies to the energy store 2. In addition, it is also possible forthe fuel cell 1 and the energy store 2 to be connected to one another,for example for a charging operation.

FIG. 2 shows a modification of the hybrid drive shown in FIG. 1, withidentical parts being identified by identical reference symbols. Incomparison with FIG. 1, a DC/DC converter 15 is additionally provided inthe line 13. The use of such a DC/DC converter 15 has the advantage thatit is possible to charge and discharge the energy store 2 (state {circlearound (2)}a or {circle around (3)}a, see below) irrespective of thevoltage and of the load state of the fuel cell 1, because the voltagesof the two circuits 5 a, 5 b can be tuned to one another by the DC/DCconverter 15. Thus, it is also possible to charge the energy store 2 viathe fuel cell 1 when the vehicle is at a standstill (state {circlearound (8)}a, see below)

TABLE 2 Possible states of the hybrid drive shown in FIG. 2 State/switchposition S6 S9 S11 S12 {circle around (1)} Driving with FC, AL on FC 10/1 0 0 {circle around (2)} Driving with FC, AL on store 1 1 1 0 {circlearound (2)}a Driving with FC, store charging 1 1 0/1 1 {circle around(3)} Driving with store, AL on store 0 1 1 1 {circle around (3)}aDriving with FC and store 1 1 0/1 1 {circle around (4)} Braking, AL onFC 0 0/1 0 0 {circle around (5)} Braking, AL on FC, store charging 0 1 01 {circle around (6)} Braking, AL on store, store charging 0 1 1 1{circle around (7)} Braking, AL on generator 0 0 1 1 {circle around (8)}Standstill, AL on FC 0 0/1 0 0 {circle around (8)}a Standstill, AL onFC, store charging 1 1 0 1 {circle around (9)} Standstill, AL on store 01 1 0

The use of a DC/DC converter 15 naturally means a loss of efficiency.However, since the DC/DC converter 15 is arranged between the fuel cell1 and the energy store 2, but not between the fuel cell 1 and theelectric traction motor 3, these converter losses arise only when theenergy store 2 is being charged, but not during driving with the fuelcell 1. The great advantage of this arrangement, however, is that, inany load state, the individual load can be regulated by the fuel cell 1and by the energy store 2 independently of one another within thephysical limits. In addition, at full load, the maximum driving power iscalculated from the sum of the powers of the fuel cell 1 and the energystore 2. In this state {circle around (3)}a, the switching devices S6,S9 and S12 are closed, which means that both the fuel cell 1 and theenergy store 2 are connected to the electric traction motor 3. Theposition of the changeover device S11 is arbitrary. Using the DC/DCconverter 15, the voltage of the second circuit 5 b can be matched tothe voltage of the first circuit 5 a, which means that the differentvoltage levels of the fuel cell 1 and the energy store 2 do not presenta problem.

In state {circle around (2)}a, the switching devices S6, S9 and S12 arelikewise closed, which means that both the electric traction motor 3 andthe energy store 2 are connected to the fuel cell 1. The position of thechangeover device S11 is again arbitrary. However, the DC/DC converter15 is now operated such that current does not flow from the energy store2 to the electric traction motor 3, but rather from the fuel cell 1 tothe energy store 2. In addition, these switch positions correspondexactly to state {circle around (8)}a while the vehicle is at astandstill. The switching device S12 is not absolutely necessary in thisarrangement and may even be omitted if appropriate.

FIG. 3 shows a further illustrative embodiment of a hybrid drive, withidentical parts again being identified by identical reference symbols.

In this illustrative embodiment, the vehicle drive comprises twoelectric traction motors 3, 3′. In the drawing, these are arranged on acommon drive shaft 17 and may, if appropriate, also be integrated in acommon housing. However, it is also possible for two or more electrictraction motors 3, 3′ to be provided with respectively separate driveshafts 17. Irrespective of this, identical or different electricmachines, for example reluctance motors or asynchronous motors, may becombined. The electric traction motors 3, 3′ are controlled on aload-dependent basis. For low moments, preferably up to 50% of themaximum moment, driving is effected using only one of the electrictraction motors 3, 3′. The other electric traction motor 3, 3′,respectively, runs constantly at the same time without any load. Forhigher moments, both electric traction motors 3, 3′ are used, with themoment being able to be distributed over the two electric tractionmotors 3, 3′, for example by means of the control unit 14.

TABLE 3 Possible states of the hybrid drive shown in FIG. 3 State/switchposition S6 S9 S11 S12 {circle around (1)} Driving with FC, AL on FC 10/1 0 0 {circle around (2)} Driving with FC, AL on store 1 1 1 0 {circlearound (2)}a Driving with FC, store charging 1 1 0/1 1 {circle around(3)} Driving with store, AL on store 0 1 1 1 {circle around (3)}aDriving with FC and store 1 1 0/1 1 {circle around (4)} Braking, AL onFC 0 0/1 0 0 {circle around (5)} Braking, AL on FC, store charging 0 1 01 {circle around (6)} Braking, AL on store, store charging 0 1 1 1{circle around (7)} Braking, AL on generator 0 0 1 1 {circle around (8)}Standstill, AL on FC 0 0/1 0 0 {circle around (9)} Standstill, AL onstore 0 1 1 0

Unlike in FIGS. 1 and 2, the switching device S12 is not in the form ofan on/off switch, but rather is in the form of a changeover device. Inaddition, a further line 16 is provided between the changeover deviceS12 and the second electric traction motor 3′. The second electrictraction motor 3′ can be selectively connected to the energy store 2 orto the fuel cell 1 by means of the changeover device S12. This meansthat, at full load, the first electric traction motor 3 can be connectedto the fuel cell 1 while the second electric traction motor 3′ can beconnected to the energy store 2. Consequently, the maximum driving poweravailable is again the sum of the powers of the fuel cell 1 and theenergy store 2. In contrast to FIG. 2, however, in this case it is notnecessary to accept the converter losses when the energy store 2 isbeing charged. In this regard (state M{circle around (2)}a), while thevehicle is in motion, the first electric traction motor 3 is drivenusing the fuel cell 1 while the second electric traction motor 3′charges the energy store 2 in generator mode. However, this arrangementwithout coupling in the drive train cannot be used to charge the energystore 2 when the vehicle is at a standstill (state {circle around (8)}ain Table 2).

FIG. 4 shows a further illustrative embodiment of a hybrid drive havinga DC/DC converter, with identical parts again being identified byidentical reference symbols. The fuel cell 1 is again connected to theelectric traction motor 3 by means of a line 7 provided with a switchingdevice S6. In addition, the energy store 2 is connected to theelectrical auxiliary loads 4 by means of a line 10 provided with aswitching device S9. Furthermore, two lines 8, 13 branch off from theline 10 between the switching device S9 and the electrical auxiliaryloads 4, said lines 8, 13 being selectively connectable to the line 7between the switching device S6 and the electric traction motor 3 bymeans of a changeover device S11. A DC/DC converter 15 is additionallyprovided in the line 13.

TABLE 4 Possible states of the hybrid drive shown in FIG. 4 State/switchposition S6 S9 S11 {circle around (1)} Driving with FC, AL on FC 1 0 0{circle around (2)} Driving with FC, AL on store 1 1 1 {circle around(2)}a Driving with FC, store charging 1 1 1 {circle around (3)} Drivingwith store, AL on store 0 1 1 {circle around (3)}a Driving with FC andstore 1 1 1 {circle around (4)} Braking, AL on FC 1 0 0 {circle around(5)} Braking, AL on FC, store charging 1 1 1 {circle around (6)}Braking, AL on store, store charging 0 1 0 {circle around (7)} Braking,AL on generator 0 0 0 {circle around (8)} Standstill, AL on FC 1 0 0{circle around (8)}a Standstill, AL on FC, store charging 1 1 1 {circlearound (9)} Standstill, AL on store 0 1 0

In driving mode, a plurality of switching states are again provided. Instate {circle around (1)}, the switching device S9 is open, which meansthat the energy store is decoupled from the circuits 5 a, 5 b. Theswitching device S6 is closed, which means that the electric tractionmotor 3 is driven using the fuel cell 1. The electrical auxiliary loads4 are also supplied by the fuel cell 1, with the changeover device S11preferably being in position 0, that is to say without the DC/DCconverter 15 interposed. This enables unnecessary converter losses to beavoided.

In state {circle around (3)}, the switching device S6 is open, whichmeans that the fuel cell 1 is decoupled from the circuits 5 a, 5 b. Theswitching device S9 is closed, which means that the electric tractionmotor 3 is driven using the energy store 2. The electrical auxiliaryloads 4 are also supplied by the energy store 2. The changeover deviceS11 is preferably in position 1, which means that the DC/DC converter 15can be used to set the voltage required for the electric traction motor3 irrespective of the voltage of the energy store 2. In the otherdriving states {circle around (2)}, {circle around (2)}a, {circle around(3)}a, the switching devices S6 and S9 are closed in each case, whichmeans that both the fuel cell 1 and the energy store 2 are connected tothe circuits 5 a, 5 b. The changeover device S11 is in position 1, whichmeans that the two circuits 5 a, 5 b are connected by means of the line13 and the DC/DC converter 15 arranged therein. Control of the DC/DCconverter 15, which, as in the other illustrative embodiments, is alsopreferably carried out by the control unit 14, governs whether theenergy store 2 is charged by the fuel cell 1 when the vehicle is inmotion (state {circle around (2)}a), whether the electric traction motor3 or the electrical auxiliary loads 4 are supplied by the fuel cell 1 orby the energy store 2 (state {circle around (2)}), or whether, at fullload, the electric traction motor 3 is supplied both by the fuel cell 1and by the energy store 2 (state {circle around (3)}a), and consequentlythe maximum driving power is again determined by the sum of the energycontents of the fuel cell 1 and the energy store 2. In addition, theDC/DC converter 15 again enables the energy store 2 to becharged/discharged when the vehicle is in motion irrespective of thevoltage or of the load state of the fuel cell 1.

During braking, the electrical auxiliary loads 4 can be supplied by thefuel cell 1 (state {circle around (4)}), in which case the switchingdevice S6 is then closed and the switching device S9 is open. Thechangeover device S11 is in position 0, which means that the DC/DCconverter 15 is decoupled. If, in addition, the energy store is to becharged (state {circle around (5)}), the changeover device S11 isswitched to position 1 and the switching device S9 is closed. The DC/DCconverter 15 which is now interposed enables the voltage to be set tothe level required for a charging operation.

By opening the switching device S6, the fuel cell 1 can be decoupled. Atthe same time, the switching device S9 is then closed and the changeoverdevice S11 is switched to position 0, which means that the DC/DCconverter 15 is also decoupled. In this state {circle around (6)}, theelectrical auxiliary loads 4 are supplied by the energy store 2. At thesame time, the energy store 2 can be charged without converter losses.In state {circle around (7)}, the switching devices S6 and S9 arefinally open, which means that both the fuel cell 1 and the energy store2 are decoupled from the circuits 5 a, 5 b. The changeover device S11 isin position 0, which means that the electrical auxiliary loads 4 aresupplied with current directly from the electric traction motor 3without interposition of the DC/DC converter 15.

When the vehicle is at a standstill, the electrical auxiliary loads 4can finally be supplied either by the fuel cell 1 (state {circle around(8)}) or by the energy store 2 (state {circle around (9)}), in whichcase either the switching device S6 (state {circle around (8)}) or theswitching device S9 (state {circle around (9)}) is closed. Thechangeover device S11 is again in position 0, which means that the DC/DCconverter 15 is decoupled. If, when the vehicle is at a standstill, theenergy store 2 is additionally to be charged (state {circle around(8)}a), the switching devices S6, S9 are closed and the changeoverdevice S11 is additionally switched to position 1. This means that theenergy store 2 is again connected to the fuel cell 1 by means of theDC/DC converter 15, so that controlling the DC/DC converter 15 enablesthe voltage to be set to a level which is required for the chargingoperation.

The switching states described using the illustrative embodiments andtables do not represent any conclusive listing. Other switching statesare also possible.

The foregoing description constitutes the preferred embodiments devisedby inventors for practicing the invention. It is apparent, however, thatthe invention is susceptible to modification, variation and change thatwill be obvious to those skilled in the art. Inasmuch as the foregoingdescription is intended to enable one skilled in the pertinent art topractice the invention, it should not be construed to be limited therebybut should be construed to include such aforementioned obviousvariations and be limited only by the proper scope or fair meaning ofthe accompanying claims.

What is claimed is:
 1. A hybrid drive system for a motor vehiclecomprising: a fuel cell; an energy store; a first electric tractionmotor, an auxiliary electrical load; a first circuit operable at a firstvoltage and adapted to selectively supply said first traction motor andsaid auxiliary electrical load with electrical energy, said firstcircuit includes a first switching device for selectively connectingsaid fuel cell and said first traction motor, said first circuit alsoincludes a second switching device for selectively connecting said fuelcell to said auxiliary electrical load; said first circuit capable ofconnecting said fuel cell, said first traction motor, and said auxiliaryelectrical load, a second circuit operable at a second voltage andadapted to selectively supply said first traction motor and saidauxiliary electrical load with electrical energy, said second circuitincludes a third switching device for selectively connecting said energystore and said first traction motor, said second circuit also includes afourth switching device for selectively connecting said energy store tosaid auxiliary electrical load; said second circuit capable ofconnecting said energy store, said first traction, motor, and saidauxiliary electrical load; and a device for controlling said first,second, third, and fourth switching devices.
 2. The invention as setforth in claim 1, wherein said first voltage can differ from said secondvoltage.
 3. The invention as set forth in claim 1, wherein said secondand fourth switching devices are a single changeover device.
 4. Theinvention as set forth in claim 2, wherein said first circuit includes afirst line disposed between said fuel cell and said first traction motorand a second line disposed between said fuel cell and said auxiliaryelectrical load, wherein said first switching device is disposed in saidfirst line, said second line branches off from said first line betweensaid fuel cell and said first switching device.
 5. The invention as setforth in claim 4, wherein said second circuit includes a third linedisposed between said energy store and said first traction motor, saidsecond circuit also includes a fourth line disposed between said energystore and said auxiliary electrical loads, said third switching deviceis disposed in said third line, said fourth line includes a fifthswitching device, said third line connects said second circuit to saidfirst circuit and branches off from said fourth line between saidauxiliary electrical loads and said fifth switching device.
 6. Theinvention as set forth in claim 5, wherein said second and fourthswitching devices are a single changeover device such that saidauxiliary electrical loads can be selectively connected to said secondline or said fourth line.
 7. The invention as set forth in claim 6,further comprising a DC/DC converter disposed between said thirdswitching device and said energy store.
 8. The invention as set forth inclaim 6, further comprising a second traction motor and a fifth linedisposed between said second traction and said third switching device,said third switching device being a changeover switch such that saidsecond traction motor can be selectively connected to said first circuitor said second circuit.
 9. The invention as set forth in claim 2,wherein said first circuit includes a first line and a second line, saidfirst switching device being disposed in said first line between saidfuel cell and said first traction motor, said second circuit includes afourth line having a fifth switching device disposed therein betweensaid energy store and said auxiliary electrical loads, a third linebranches of from said fourth line between said fifth switching deviceand said auxiliary electrical loads, said first line being capable ofbeing selectively connected to said second line or to said third line.10. The invention as set forth in claim 1, wherein said energy store isa battery.
 11. The invention as set forth in claim 1, wherein said firsttraction motor can be operated as a generator.
 12. The invention as setforth in claim 8, wherein said second traction motor can be operated asa generator.
 13. A method for operating a hybrid drive system for amotor vehicle comprising: providing a fuel cell; providing an energystore; providing an electric traction motor; providing an auxiliaryelectrical load; providing a circuit having a plurality of switchesselectively interconnecting said fuel cell, said energy store, saidtraction motor, and said auxiliary electrical load; powering saidtraction motor with said fuel cell and separately powering saidauxiliary electrical loads while in a first load driving state; poweringsaid traction motor and said auxiliary loads with said fuel cell whilein a second load driving state; powering said traction motor and saidauxiliary loads with said energy store at a third load driving state;wherein said first load is greater than said second load and said secondload is greater than said third load.
 14. The invention as set forth inclaim 13, further comprising: powering said auxiliary loads by said fuelcell while in a first standstill state; powering said auxiliary loads bysaid energy store in said second standstill position while said fuelcell is disabled.
 15. The invention as set forth in claim 13, whereinsaid traction motor can also act as a generator.
 16. The invention asset forth in claim 15, further comprising charging said energy storewith said traction motor while in a braking state.
 17. The invention asset forth in claim 15, further comprising: powering said auxiliaryelectrical loads with said fuel cell and charging said energy store withsaid traction motor while in a first braking mode; powering saidauxiliary electrical loads with said energy store and charging saidenergy store with said traction motor while in a second braking mode,said second braking mode being more severe than said first braking mode.18. The invention as set forth in claim 17, wherein said second brakingmode providing a greater deceleration of said vehicle than said firstbraking mode.
 19. The invention as set forth in claim 17, furthercomprising powering said auxiliary electrical loads with said generatorin a third braking state.
 20. The invention as set forth in claim 13,further comprising operating said fuel cell at a first voltage andoperating said energy store at a second voltage.
 21. The invention asset forth in claim 20 wherein said first voltage can be different fromsaid second voltage.
 22. The invention as set forth in claim 21, furthercomprising providing a DC/DC converter and charging said energy storeand powering said traction motor with said fuel cell while said vehicleis in a fourth driving state.